1. Field
This invention generally relates to the field of temperature sensors in integrated circuits.
2. Background
When temperature is measured in an integrated circuit, a semiconductor junction is often used in the process. By manipulating the currents and the current densities through the junction, changes in voltage can be measured across the junction. A change in voltage at two current densities across the junction can be measured and used by a temperature sensor to calculate temperature. Most junctions employed for this purpose are parasitic vertical p-n-p silicon based transistors. However, it should be appreciated that n-p-n transistors or even diodes may be used instead.
The classic transistor equation determines a change in the base emitter voltage (ΔVBE) for a p-n-p transistor as follows:
                              Δ          ⁢                                          ⁢          Vbe                =                  η          ⁢                                    κ              ⁢                                                          ⁢              T                        q                    ⁢                      ln            ⁡                          (                                                I                  CN                                                  I                                      C                    ⁢                                                                                  ⁢                    1                                                              )                                                          (        1        )            where η is a non-ideality constant substantially equivalent to 1.00 or slightly more/less, κ is the well known Boltzmann's constant, q is the electron charge, T is the temperature in Kelvin, IC1 and ICN are collector currents that are present at the measurement of a first base-emitter voltage and a second base-emitter voltage respectively.
There are two basic types of temperature sensors that utilize the concept of the diode equation: “diode mode” sensors and “transistor mode” sensors. Diode mode sensors operate on the assumption that a ratio of collector currents tends to be relatively equivalent to a ratio of known emitter currents (IE). Hence, for a diode mode sensor, the diode approximation of the transistor equation (or “diode equation” for short) is approximated by:
                                          Δ            ⁢                                                  ⁢            Vbe                    =                      η            ⁢                                          κ                ⁢                                                                  ⁢                T                            q                        ⁢                          ln              ⁡                              (                                                      I                    EN                                                        I                                          E                      ⁢                                                                                          ⁢                      1                                                                      )                                                    ;                              where            ⁢                                                  ⁢                                          I                                  C                  ⁢                                                                          ⁢                  1                                                            I                                                      C                    ⁢                                                                                  ⁢                    2                                    ⁢                                                                                                                      =                                    I                              E                ⁢                                                                  ⁢                1                                                    I                              E                ⁢                                                                  ⁢                2                                                                        (        2        )            
As process geometries have decreased, the beta (ratio of collector current over base current) has been shown to vary as much as ten percent or more between two known emitter currents for p-n-p transistors. Transistor mode sensors, which utilize TruTherm™ technology invented by National Semiconductor, have evolved as a consequence. Transistor mode sensors do not operate on the assumption that collector current is equivalent to emitter current. Therefore, they will either measure the actual collector currents directly or they will employ a feedback circuit to drive the collector currents to a known ratio. As a result, transistor mode sensors use the transistor equation shown in Equation 1. Examples of the TruTherm™ invention, methods, and techniques can be found in U.S. patent application Ser. No. 10/865,609, filed Jun. 9, 2004, by Mehmet Aslan, entitled “BETA VARIATION CANCELLATION IN TEMPERATURE SENSOR,” hereinafter referred to as the “TruTherm™ application,” which is hereby incorporated by reference herein in its entirety.
In both diode mode and transistor mode sensors, a problem arises in measuring the voltage across the junction, because the actual voltage across the junction is never measured due to the fact that error terms are introduced by series resistances in the measurement path to and from the junction. The exemplary circuit diagram of FIG. 1 illustrates one temperature measuring circuit that experiences this problem of measurement-induced error. In FIG. 1, a temperature sensor supplies a current to the emitter of a PNP transistor, and then receives an input current from the base of the same transistor. A base-emitter voltage is generated across the base-emitter junction of the transistor. However, due to the series resistance of the measurement lines, the temperature sensor actually measures a slightly different voltage than what is present across the base-emitter junction of the transistor. The series resistance is represented by resistor RE in series between the temperature sensor and the emitter of the PNP transistor and resistor RB in series between the base of the PNP transistor and the temperature sensor. The presence of these series resistances introduces error.
In the past, especially in integrated circuit production techniques at the 0.09 micron level and larger, this type of measurement-induced error could be ignored by a temperature sensor because accuracy needs were not as stringent. However, at smaller circuit production techniques, this error becomes larger and must be dealt with. A typical way to deal with this was generally to add an offset—either a resistance offset, a temperature offset, or a software offset that helps compensate for the error that is induced by the measurement. In one case, the amount of offset would be determined by simply multiplying a typical resistance of the circuit by a typical current through the circuit. In another case, the amount of offset would be determined by multiplying a typical resistance by the actual current. In either case, the offset only works in conditions where the error term has no significant temperature dependency. In the past, there was only a very small temperature dependency in the measurement errors. However, at smaller integrated circuit sizes, starting at around 65 nanometers, there is a large temperature dependence in the resistances induced by measurement. As a consequence, simply dealing with these resistances through the use of some sort of offset does not yield an accurate temperature measurement at a variety of temperatures, and therefore the overall system accuracy of a temperature system suffers. A further problem exists in that some portions of the error term are non-obvious, and thus hard to identify.
There are also well-known techniques for dynamically canceling the effects of this series resistance on a real time basis. These techniques are only suitable for cases when the sensing junction is an actual diode or a transistor that substantially behaves like a diode, i.e. has high and constant current gain. For the small geometry processes it has been shown that the temperature sensing transistors do not behave like simple diodes, hence making these dynamic resistance correction techniques largely useless.